The present invention generally relates to brazing processes and materials for repairing components that operate at high temperatures. More particularly, this invention relates to a process of filling cracks and other defects in hot gas path components of gas turbines, including through-thickness cracks in relative thin components such as the impingement plate of a gas turbine engine.
Hot gas path components of gas turbines are typically formed of nickel, cobalt or iron-base superalloys with desirable mechanical and environmental properties for turbine operating temperatures and conditions. A particular example is an impingement plate that protects the aft end of a combustion liner cap assembly, through which fuel nozzle assemblies extend into the combustor of a gas turbine engine. Impingement plates may be equipped with effusion cooling holes to promote the ability of the impingement plate to serve as a radiation shield for the cap assembly. Impingement plates are typically thin, one nonlimiting example being about 0.092 inch (about 2.3 mm) in thickness, with the result that through-cracks can develop in the thermally hostile environment near the combustor. Various methods have been used to fill these cracks, including brazing and welding techniques.
As known in the art, brazing repair techniques are performed at temperatures lower than the melting point of the base metal of the component being repaired. Brazing performed on superalloy components has typically involved the use of braze materials in pliable forms such as pastes, putties, slurries, and tapes, as evidenced by commonly-assigned U.S. Pat. Nos. 6,187,450 to Budinger et al., 6,530,971 to Cohen et al., and 7,279,229 to Budinger et al. Brazing techniques using sintered preforms have also been proposed for applying wear resistant materials on bucket surfaces, as taught in commonly-assigned U.S. Pat. No. 7,335,427 to Sathian, and for surface buildup and hardfacing as taught in commonly-assigned U.S. Published Patent Application No. 2007/0154338 to Sathian et al. Brazing pastes, putties, slurries, and tapes generally contain metal particles in a binder that adheres the metal particles together and to the surface(s) being brazed, and then burns off during the brazing operation. The metal particles are typically a mixture of two or more different alloys, one of which contains a melting point depressant (for example, boron or silicon) to achieve a lower melting point than the remaining metal constituents of the braze material. During brazing, the lower melting particles melt to form a liquid that fills voids between the higher melting particles and, on solidification, bonds the high melting particles together and to the substrate material. Difficulties associated with such braze materials include the difficulty of consistently using optimal quantities of the braze material, accurately placing the braze material, and accurately shaping and sizing the braze material for the area being brazed. Other shortcomings may include low densities and excessive porosity and voids created during binder burn-off, resulting in poor mechanical properties for the resulting brazement.
In the case of repairing impingement plates equipped with effusion cooling holes, additional shortcomings include the difficulty of filling small through-cracks in a relatively thin wall section, as well as larger through-openings, in which case the braze material must be sufficiently viscous so that the material doesn't flow away from the area being brazed prior to and during brazing, yet must be capable of completely infiltrating cracks of various widths.